When using the consecutive(polygon/polyline) line drawing function, after you undo, the 'drawing from' coordinates are not undone: so the next use of it draws from that point which is no longer relevant.
To illustrate:

A B

C D

say I draw lines from A to C to D to B. then I decide the D->B line was wrong and I want to draw D->A.
If I undo and then draw to A, the line will be drawn from B rather than D.

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